An apparatus manufactured by the Eskofot A/S of Bellerup, Denmark allows an exposure of a metal sheet inside a closed box. The metal sheet is placed in a lid and retained by means of vacuum means, whereafter the lid is turned 180.degree. in such a manner that the surface to be exposed faces a light source in the bottom of the box. The distance between the surface and the light source is, however, not sufficiently large for providing a substantially uniform exposure over the entire surface to be exposed. U.S. Pat. No. 3,357,335 discloses a somewhat similar apparatus.
It is also known to suspend the light source in the ceiling and to draw a curtain for the light source during the exposure. The large distance to the light source renders it possible to achieve an almost uniform exposure of the surface to be exposed. Such systems are, however, large and unmangeable, and the large light sources arranged below the ceiling generate much waste resulting in dry air and static electricity.